1. Field of the Invention
The present invention relates to a belt driving apparatus that rotationally drives an endless belt, and an image forming apparatus that is provided with the belt driving apparatus, such as a copier, a facsimile, and a printer.
2. Description of the Related Art
As an image forming apparatus that uses an electrophotography method, an image forming apparatus employing a so-called intermediate transfer method has been known. This type of image forming apparatus creates a full-color toner image on an intermediate transfer belt (ITB) serving as an endless belt.
In addition, as an image forming apparatus intended for high speed operations, there is an image forming apparatus that detects a deviation of the endless belt and controls an alignment of a stretching roller, so that a position of the stretching roller along a longitudinal direction (an axial direction) is maintained within a substantial constant range.
JP-A-2001-355693 discloses a belt apparatus configured in such a manner that a friction force between the endless belt and a driven roller, which serves as the stretching roller, is smaller than a friction force between the endless belt and a driving roller, which serves as the stretching roller. As a result, deformation such as waving and wrinkling can be prevented.
In such a belt apparatus, a so-called “belt deviation” may occur. In other words, the intermediate belt is deviated toward either one of both end portions of the stretching roller at the time of being driven. This may be caused by, for example, variations in accuracy of an outer diameter of the stretching roller, variations in accuracy of mutual alignments between rollers, and the like. In the belt driving apparatus employing a belt deviation control scheme to prevent the belt deviation, the belt deviation control becomes inoperative when the intermediate transfer belt slips on a steering roller. In order to avoid this, a friction coefficient between the steering roller and the intermediate transfer belt is preferably set greater.
Therefore, when the configurations disclosed in JP-A-2001-355693 are applied to the belt driving apparatus of the belt deviation control scheme, the driving roller, rather than the driven roller, should preferably function as the steering roller.
As an image forming apparatus of a vertical path scheme, which is beneficial to make the apparatus compact, there is a type of image forming apparatus configured in such a manner that a pressure can be applied to an outer secondary transfer roller and thus on an inner secondary transfer roller positioned on the other side of the steering roller through the intermediate transfer belt. In such an image forming apparatus, a cleaning blade may be provided so as to be in contact with the intermediate transfer belt supported by the steering roller, thereby to retrieve residual toners remaining on the intermediate transfer belt. When such a cleaning blade is used, a pressure applied to the outer secondary transfer roller and thus on the inner secondary transfer roller becomes greater than a pressure applied to the steering roller from the cleaning blade.
Here, a force caused between the stretching roller and the intermediate transfer belt is obtained by multiplying the friction coefficient between the stretching roller and the intermediate transfer belt with the normal force.
The normal force corresponds to a component force of a tensional force applied to the intermediate transfer belt and a force applied from outside, along a radius direction of the stretching roller.
Therefore, in a case where the configurations of JP-A-2001-355693 are applied to the apparatus employing the belt deviation control scheme, even when a friction coefficient of the driven roller with respect to the intermediate transfer belt may be smaller than a friction coefficient of the driving roller that also functions as the steering roller with respect to the intermediate transfer belt, the following may occur.
Namely, in some cases of external forces applied respectively on the driving roller and the driven roller, a belt restraining force of the driven roller may be beyond a corrective capability produced by steering the driving roller.
If such a situation happens, the belt deviation control, which is performed by steering the driving roller, becomes insufficient. As a result, deformation such as waving and wrinkling may occur, or malfunction caused from a fully deviated belt may occur. Especially, after images are repeatedly created in the image forming apparatus, a surface of the driving roller becomes tainted with toners scattered from a transfer cleaner, which decreases the friction coefficient with respect to the intermediate transfer belt, and thus may make the malfunction mentioned above significant.
In addition, because a high transfer bias voltage is applied to the inner secondary transfer roller described above, a component of rubber may exude, even if only slightly, to the roller surface, so that the friction coefficient with respect to the intermediate transfer belt may be increased. Therefore, when the inner secondary transfer roller is configured as the driven roller, a belt restraining force produced by the driven roller tends to be beyond the belt deviation corrective capability due to the driving roller that also serves as the steering roller.